Essay: Reflux

These benign reflux episodes are 90% cleared by a secondary peristalsis wave and 10% cleared by saliva neutralizing the remaining substance; and tend not to pose a danger for heartburn or inflammation. Thus, 80% to 90% of reflux episodes go unnoticed or are asymptomatic. It can be considered, then, that GERD patients are categorized into two subgroups that have different clinical characteristics: 1) the erosive esophagitis (EE) group that experiences inflammation of the mucosal lining with chronic hypotensive LES; and 2), the non-erosive reflux disease (NERD) group that has the common heartburn symptom with pathological acid reflux but without evidence of mucosal damage on endoscopy.
Heartburn is the stimulation of chemoreceptors in the mucosa layer by gastric acid and bile salts where the pH is less than four; and it presents as the sensation of pain or burning in the thoracic heart region. NERD is estimated to comprise about 50% to 70% of the GERD population, and the pathogenic risks of going from profiles of NERD to EE to BE are assumed to be very low across all categories and is still under investigation. Essentially, once found to be in either of the three categories, the patient state will remain in that specific category for the remainder of their lifetime. The patient profile for both NERD and erosive esophagitis are similar in that the mean age at diagnosis is about 50, but a 60/40 split for women to men comprises the NERD group, while 59% of the EE group is men. EE patients also present with a higher body mass index and a higher prevalence of hiatal hernia.
It is when the episodes of reflux become more frequent and protracted (longer than five minutes) that the normal esophageal clearance mechanisms begin to lose their effectiveness and lead to esophagitis and BE. Unsurprisingly, comparing healthy individuals to NERD, esophagitis, and BE produces an escalating increase in the number of reflux events and longer exposure periods to acids less than pH 4.0 across the sequence. The persistence of acid in the range of pH 2.0 to 4.0 combined with pepsin induces damage to the mucosal lining, causing the presence of dilated intercellular spaces that are evidence of the first lesion. The dilated spaces between the cells allow for the penetration of acid deeper into the mucosal lining, which compromises the mucosal resistance to acid. Deeper acid penetration also activates or recruits more peripheral nociceptors to transmit pain signals. Esophageal damage is assessed according to the Los Angeles
Classification of Esophagitis :
• Grade A: One (or more) mucosal break no longer than 5 mm that does not extend between the tops of two mucosal folds
• Grade B: One (or more) mucosal break more than 5 mm long that does not extend between the tops of two mucosal folds
• Grade C: One (or more) mucosal break that is continuous between the tops of two or more mucosal folds but which involve less than 75% of the circumference
• Grade D: One (or more) mucosal break which involves at least 75% of the esophageal circumference
The greatest chances for reflux occurring are 30 to 60 minutes after eating a meal, during bending over, lying down, and during sleep. Pain may also be caused by mechanical means like esophageal distention and peristalsis motility disorders. Numerous causes to the sensation of pain can obfuscate the true nature of GERD, though, and may mean a combination of anatomic and physiologic disturbances. Other symptoms from GERD may include dysphagia (difficulty swallowing), laryngitis, pharyngitis, asthma, chronic cough, or dental erosion. Reflux reaching all the way back up the esophagus and into the pharynx may cause inflammation in the pharyngeal tissue or erode teeth enamel; and if it escapes into the larynx it can irritate the vocal cords and trachea.
Malfunctioning of the LES plays a key role in the reflux of stomach fluid in the esophagus. Normally, the LES’s resting pressure is 15 mm Hg to 30 mm Hg greater than that of the intra-abdominal pressure and will adequately inhibit the reflux of stomach fluids. The pressure at the LES does not stay at a constant rate, though, and will differ depending on the body position, breathing, movement, and time of day; where it is low during the day and higher at nighttime. The LES pressure also falls under the influence to different outside compounds (Table 2) like medications, hormones, and food chemicals.
Table 2. Compounds that irritate conditions within the gastric chamber.
Foods Peppermint, chocolate, caffeine, alcohol, tobacco, citrus fruits and juices, spicy foods, raw onions, tomatoes
Hormones & Neurotransmitters Secretin, cholecystokinin, glucagon, progesterone, vasoactive intestinal polypeptide, gastric inhibitory polypeptide, serotonin, dopamine
Medications Nitrates, morphine, barbiturates, diazepam, calcium channel blockers, atropine, tricyclic antidepressants, ganglion blockers
A second line of control comes into play with the left and right crural muscles of the diaphragm, which connect to the LES via the phrenoesophageal ligament. These diaphragm muscles normally aid in preventing reflux during times of heavy lifting, pregnancy, and any other causes for increased abdominal pressure. Transient LES relaxations (TLESRs) are relaxations of the LES moderated by the vagus nerve and usually caused by the buildup of gastric gases or stomach distension, as in, they are unrelated to swallowing or peristalsis. These relaxations of the LES can last anywhere between 10 to 35 seconds and reduce the pressure of the LES to levels aligned with the gastric pressure. During TLESRs, the crural muscles of the diaphragm are inhibited, releasing pressure on the sphincter opening. TLESRs are more frequent in GERD by contributing to almost 90% of reflux episodes, and hiatal hernias exacerbate the problem even further by leading to increased sphincter openings. Hiatal hernias are when a portion of the stomach pushes upwards past the diaphragm. Ninety percent of hiatal hernias are Type 1, where the LES moves upwards and pushes through the diaphragm. Rarer forms involve the squeezing of the fundus portion of stomach past the diaphragm and results in a pouch conducive for lingering stomach acid. Type 1 causes a reduction of LES pressure and length, limiting acid clearance and increasing mucosa exposure to acid compounds. Obesity and weakening of the surrounding muscles are the main causes for hiatal hernias, with increasing age contributing to the weakening of muscle structures. Older people typically have a higher body mass index, as well. The phrenoesophageal ligaments may also lose elasticity with age and contribute to the formation of the hernias. When hiatal hernias exceed two centimeters, then the incidence of esophagitis and BE is higher.
Factors that can contribute to the development of GERD involve several modes of dysfunction. Peristaltic dysfunction hinders acid clearance from the distal esophagus and comes in the forms of failure for peristaltic contraction or low-amplitude contraction waves. Dysfunction in the emptying of the stomach can occur through gastroparesis, neuromuscular diseases, pyloric dysfunction, duodenal motility disorders, and even duodenogastric reflux (DGR). DGR is the reflux of bile from the duodenum back into the stomach. Some bile reflux is normal, due to proximity, but reflux may occur in larger quantities and mix with the contents of the stomach. Upon gastric reflux, the gastric acid and bile mixture travels up into the esophagus. DGR is found to play a role in inflammation and injury of the esophagus. In those with esophagitis, DGR is much more common and means bile acid along with gastric acid are compounding injury to the mucosal lining (Fig. 9). With patients exhibiting the Los Angeles esophagitis rating system from A-D in increasing mucosal breaks, DGR has been in found in 67, 68, 80, and 100% of patients in those categories, respectively. This finding establishes a clear link between the interplay of DGR, bile acid, and esophagitis leading up to metaplastic change. Other factors are Zollinger-Ellison syndrome (where acid is secreted in above normal amounts), ulcers or strictures that obstruct the gastric outlet, and connective tissue disorders like scleroderma.
Figure 9. Comparing acid-bile profiles among different GERD subgroups shows that having acid and bile together leads to greater risk for complicated BE, or the precancerous condition of dysplasia.
Stomach Acids & Bile
As has been mentioned previously, stomach acids and bile acids play a central role in the development of metaplastic BE tissue. Under normal conditions, stomach acids are pivotal to the breakdown of foods and the proper functioning of the digestive system. Our nervous system is highly sensitive to stimuli involving food, where the sight, smell, touch, and taste of food trigger the acid regulatory pathways in the stomach: acetylcholine, gastrin, and histamine. The gastric glands of the stomach lining regulate the acid pathways and produce numerous substances like mucus, gastrin, pepsinogen, histamine, and intrinsic factor. Mucus secreting cells are located superficially in the glands and are meant to protect the stomach lining from its very own acid through a bicarbonate buffer. The cells located deeper in the glands are parietal cells, which secrete hydrochloric acid (HCL) based on surface receptor activation. Those receptors are sensitive for histamine, acetylcholine, and gastrin; and will cause a flood of HCL that can lower the pH of the gastric fluid to 1 or less. Upon a stimulus, gastrin is released from G cells in the antrum of the stomach. The gastrin travels through the blood stream to gastrin receptors on parietal cells in the gastric body and fundus; and begins the chemical activation of proton pumps located on the opposite side of the cells. Parietal cells will also activate the HCL pathway through receptors for histamine and acetylcholine. Pepsinogen is released by chief cells and breaks down proteins. It becomes converted to pepsin upon exposure to low pH. If the pH becomes too low, gastrin production is shut down via a negative feedback loop, halting HCL secretion.
The scientific literature describes inflammation of the esophagus due to acid reflux that is a consequence of GERD. Continued exposure of the esophageal lining to HCL and pepsin begin the breakdown and inflammation of the mucosal lining. The pepsin will destroy proteins securing the tight junctions between cells, allowing hydrogen ions to reach deeper into the lining. Enhancing this process is that the esophagus simply lacks the necessary amount of mucus to negate the acid. Subsequent to a reflux episode, mass mitosis may occur to compensate for the cell deaths that acids cause on the esophageal lining. Basal cells may experience hyperplasia and inflammatory cells will be attracted to the area in vast quantities. They will release cytokines, or interleukins, that have an influence over the proliferation and regeneration of cells. Reactive oxygen species are also released by the immune cells that wreak havoc on cell membranes and clash with DNA to cause breaks and other deleterious effects.
Along with stomach acids, bile acids may become refluxed back into the stomach. Bile acids have received much scientific attention recently in relation to GERD and BE. It normally serves as a detergent compound that enables the absorption and transport of vitamins, nutrients, and lipids in the small intestine. Bile is an amalgamation of numerous different substances. Mainly, it is 95% water, but is also composed of bile acids, bilirubin, amino acids, steroids, heavy metals, medicine, and toxins. Bile serves to transport cellular waste from the body, like old red blood cells, hormones, and harmful lipophilic substances that cannot be excreted through urine. It also removes cholesterol from the body and helps protect against GI infection. Originating from cholesterol, the synthesis of bile in the liver is the primary pathway for cholesterol catabolism into amphipathic (hydrophobic & hydrophilic) molecules and serves to release up to half the body’s cholesterol. After being synthesized in the liver, bile collects in the gallbladder for temporary storage until food matter enters the duodenum. The food triggers the gallbladder to contract and release bile into the duodenum to combine with the food.
The current literature describes how increasing exposure to both stomach acid and bile on the esophagus is highly compounding of injury and inflammation. The composition of bile acid is roughly 40% cholic acid, 40% chenodeoxycholic acid (CDCA), and 20% deoxycholic acid (DCA). The DCA comes from bacterial conversion of cholic acid in the intestine, thus, making it a secondary bile acid. DCA is a hydrophobic compound that has destructive qualities towards the membranes of cells, causing randomized damage with increasing inflammation and severity based on concentration. Recent findings support that bile acids are a primary contributing factor in the development of BE, where they can exert greater influence than gastric acid alone. CDX2, Muc2, and BMP4 have been found in elevated levels in the esophageal cells of rats exposed to BA, and suggest that esophagitis that leads to metaplasia and dysplasia is dependent on BA. DCA has also been shown to act through an array of pathways and mechanisms, like stimulating ROS activity, which activates the nuclear factor kappa beta (NF-kB) pathway while also leading to DNA damage. This means that the response of intestinal cells and BE cells to DCA is incredibly different, despite similar morphologies. The NF-kB pathway is oriented towards ensuring cell survival and anti-apoptosis; properties that would enable the survival of BE tissue subjected to frequent reflux episodes. It is theorized that the metaplastic specialized tissue of BE has resistance to apoptosis so that the repairing of damaged DNA caused by reflux may occur. However, the anti-apoptotic quality has a caveat: it is also a factor in the development of EAC, where a mutation can be passed on to daughter cells.
Dysplasia
The literature has seen extensive studies on dysplasia. It often follows the initial metaplastic development of specialized intestinal epithelium and means an identifiable change in the structure of the cell. Dysplasia, in general, is defined as precancerous neoplastic epithelium growth that is isolated in the basement membrane of the associated gland. The observed change in cells is characterized by a larger cell nucleus, which helps to identify cytologic atypia. Termed hyperchromasia, the increased DNA within the nucleus allows for it to draw up more staining chemicals and the nucleus will appear darker under the microscope. The degree of the cytologic atypia is what can separate dysplasia into two subgroups of identification, low-grade and high-grade. Low-grade dysplasia presents as varying hyperchromasia, a small degree of nuclear clumping, and abnormal nuclear contours. The cells still exhibit polarity where the nucleus is oriented towards the basal end. Goblet cells are nominal compared to regular metaplastic BE tissue. On the other end, high-grade dysplastic cells feature more extreme atypia than low-grade and have more pronounced architecture abnormalities. The villiform surface of the tissue is much more evident with cribriform glands. The nuclei of the cells have increased hyperchromasia along with less cytoplasm space and loss of nuclear polarity. Abnormal mitosis of the cells at the epithelial surface occurs (when it usually does not), and the cells never completely mature as the surface cells highly resemble those in the bottom of the crypts. High-grade dysplasia is the immediate form of lesion before esophageal adenocarcinoma, yet the cells do not have the capability to penetrate deeper into the epithelial tissue layers or metastasize. Diagnosis of dysplasia requires endoscopy and histologic examination. Using narrow-band imaging during endoscopy, irregular vasculature and mucosal patterning abnormalities help to identify dysplastic tissue. Some cases of dysplasia are deemed indefinite if it is unclear whether the enlargement of nuclei and atypia are due to tissue repair. Even when having experience diagnosing lesions of dysplasia, consensus amongst doctors can be a toss-up in identifying histological tissue states, where, when shown three images of a biopsied lesion, pathologists voted: No dysplasia – 3, Indefinite – 3, Low-grade – 8, High-grade – 9, and invasive carcinoma – 1.
Adenocarcinoma
Esophageal adenocarcinoma arises through the long-term continued exposure of reflux acids to the esophageal lining. As mentioned previously, adenocarcinoma follows high-grade dysplasia and consists of varying hyperchromasia and irregularly shaped nuclei clumping together into undifferentiated cells. Generally, cancer cells and tumors display a shared list of characteristics that sets a base of understanding for any malignant lesion. Eight features have been identified as keys to the proliferation of cancer :
1. Continued cell signaling
2. Escaping growth-limiting factors
3. Resistance to apoptosis
4. Achieving cellular immortality
5. Activating blood vessel growth
6. Spreading locally and to distant segments of body
7. Conversion of energy metabolism for needs
8. Harmony with immune system
For EAC, the literature describes the accumulation of genetic and epigenetic alterations that lead to the development of the malignant tissue. The genetic changes found in BE tissue addresses somatic mutations that can be passed on to daughter cells due to the anti-apoptotic pathway of NF-kB. Loss of heterozygosity in BE has been found on tumor suppressor gene loci 17p for p53 and 9p for CDKN2A, meaning the loss of that gene’s corresponding allele on the other chromatid. p53 is intimately involved in the G1/S checkpoint and can call in for DNA repair or signal for apoptosis if repair is futile.
CDKN2A is part of the cell-cycle regulation dynamic. The mutation of p53 is known to be a later event compared to uncomplicated BE, where over two thirds of patients with high-grade dysplasia and EAC have been found to harbor mutation, usually seen with tetraploidy as the genome careens towards further dysfunction. Mutations to tumor suppressor genes have been found in nondysplastic BE patients, but conflicting data leave that development profile unclear while whole exome and genome research continues to unravel genome alterations.
The control of the genetic environment regulates the state of the chromatin and access to DNA sites where transcription factors and other influential proteins can position for activation. Epigenetic mechanisms involved in this control include :
• The methylation of cytosine bases in DNA sequences abundant in CG dinucleotides (CpG) – the use of sequencing points towards a general tendency for more hypomethylation of CpG sites in both BE and EAC, but hypermethylation also presents
• Posttranslational alteration to the histone packaging proteins
• MicroRNAs (miRNA) and noncoding RNA – miRNA 21 is upregulated in BE and EAC, 34 other differentially expressed miRNAs
• Nucleosome positioning
Figure 10. Different staging categories for the distinction of esophageal adenocarcinoma in surrounding mucosal layers.
EAC staging uses 2010 American Joint Committee on Cancer guidelines. Under these guidelines, high-grade dysplasia is classified as an in situ carcinoma, or Tis (Fig. 10). T1a category tumors are limited to the mucosal layers, with spreading to the third layer, the muscularis mucosa, the extent of the superficial tumor development. Also to note, at this stage the regional lymph system becomes a component for possible tumor interaction. A designation of N0 means no metastasis to the local lymph nodes, whereas an N1 means one to two nodes show tumor characteristics. Increasing from N1 through N3 means more regional lymph nodes are recruited into the metastasis of tumor tissue. T1b tumors have moved beyond the mucosal layers and into the submucosa. At T2 stage, the tumor is within the muscular layers, either the circular or longitudinal. T3 reaches the outermost adventitia and T4 is when the tumor has started to invade adjacent local tissues, like the aorta, diaphragm, vertebrae, or trachea. If the tumor reaches higher levels of N2, N3, or T4; the patient is at higher risk for M1 metastasis to distant regions of the body due to having advanced penetration into vessel network structures.